The GT64260 is the PPC equivalent of the mips 64240. I'm pretty sure
our PPC guys did some work with the 64260 and they might have one of the
serial ports working as a simple uart. Check the community PPC tree. If
you don't find anything there, post to the ppc mailing list to see what
people have done with the 64260. Pretty much everything should be
portable to the 64240 and you might get feedback on hardware bugs and
workarounds.
Pete